EACH OTHER" to a simple "PRIMARY - SECONDARY" topology
that contains only one backup per server. Thus as LOAD
increases partitions of ever diminishing sizes are created.
Since back-ups happen only within a partition the system
can scale well (also administrators can specify MAX_SERVERS_IN_CLUSTER
and MIN_SERVERS_IN_CLUSTER).
The document is divided into
1. Concept
2. Case of Failover
3. Case of Node Added
4. Case of excessive load on a Server
while the attachment has topics 1 and 2 , i am still writing the
others. I though that topic-2 was particularly complex so i though
of putting it up on the mailing-list for all to review and
digest it till the relatively simple (i guess) topics 3and4 are ready
thanks
-hb
-----Original Message-----
From: Jules Gosnell [mailto:[EMAIL PROTECTED]
Sent: Thursday, October 23, 2003 11:55 PM
To: Bhagwat, Hrishikesh; Geronimo Developers List
Cc: Biswas, Vivek
Subject: Re: [Re] Web Clustering : Stick Sessions with Shared Store -
curr ent state of play.
I agree on needing to be able to run without a db.
If you think about it, a db is just a highly specialised node in the
cluster. It is much simpler, and therefore maintainable, if all nodes in
a Geronimo cluster are homogeneous. We can't lose the db your business
data lives in, but if we can avoid adding another for session
replication it might be of advantage.
To this end, my design should work without a db. You just tune it so
that passivation never occurs - i.e. unlimited active sessions. You
trade of in-vm space against db space.
Likewise, if you were an advocate of the 'shared store' approach, you
should be able to constrain the web container to keep '0' active
sessions in memory, but passivate everything immediately to the db.
So, yes, I'm sure we are on the same page.
Jules
Bhagwat, Hrishikesh wrote:
hi Jules,
I am happy that we are converging, in that, the following approach does very
well cover some of the main goal that i was trying to achieve when i wrote my
first proposal. I have marked those points below as [hb-X] with brief comments.
Thus keeping the "Goal of the Architecture" the same I would like to propose a new
scheme for "replication". While initially, I was keen on having NO DATA EXCHANGE BETWEEN
SERVERS but rather on having them all just use the shared store to persist and retrieve session
data, you were interested in the contrary. Your approach (mentioned in section 2 of http://wiki.codehaus.org/geronimo/Architecture/Clustering) is about Session object exchange
between servers which are clustered and about clusters that are partitioned (stat/dynamically).
I think the discussion there stops with some issues that you think that may arise with that
kind of a scheme.
After Vivek Biswas first pointed out to me, I have been feeling increasing uncomfortable with the idea of
Geronimo being DEPENDENT on an EXTERNAL system like a Database for implementing its web clustering.
Some ppl on the mailing list have spoken about performanced issues with the DB approach but i
think with techniques like asny-write-to-DB etc such problems can be circumvented. Thus though I
still believe that its a solution that can solve the problem, I am not comfortable with the usage of
an external system to aid in clustering. I dont know of any GOOD (hi-av) DBs that comes for FREE. This
mean that even if Geronimo is available for FREE it cant be used without purchasing a DB. The solution
as a whole, is then, not truely FREE-of-cost.
I have been since then looking at many alternatives ... like the Jetty implemented solution of
having all m/c in one cluster to a 1Primary:1Secondary solution.
Presenly I have something cooking. As I come close to working on details I find it quite similar to
your original solution of dynamic patitioning. This is what again convinces me that we seem to converge.
my porposal in to exchange note and to jointly come up with a detailed design.
Do let me know your thoughts on that .... I am tring to complete a document on
this new schema ASAP and will
mail you the same.
thanks
- hb
-----Original Message-----
From: Jules Gosnell [mailto:[EMAIL PROTECTED]
Sent: Thursday, October 23, 2003 3:44 AM
To: [EMAIL PROTECTED]
Subject: Re: [Re] Web Clustering : Stick Sessions with Shared Store -
current state of play.
Guys,
since this topic has come up again, I thought this would a useful point
to braindump my current ideas for comment and as a common point of
reference...
Here goes :
Each session has one 'primary' node and n 'replicant' nodes associated
with it.
Sticky load-balancing is a hard requirement.
Changes to the session may only occur on the primary node.
[hb-1] i always intended only the OWNER NODE (remember ... i wasn't keen on having
Primary/Sec. concept so i am using this term)to make changes to the SESSION. Also
only ON CHANGE the session would be sent for persistance.
Such changes are then replicated (possibly asynchronously, depending
on data integrity requirements) to the replicant nodes.
[hb-2] My approach was about NOT USING REPLICATION AT ALL but using
a shared data store. Though I never mentioned it explicitly (bocz I
regarded this as a finer detail) ... i always intended on having an
async sys do the persistence.
If, for any reason, a session is required to 'migrate' to another node
(fail-over or clusterwide-state-balancing), this 'target' node makes a
request to the cluster for this session, the current 'source' node
handshakes and the migration ensues, after which the target node is
promoted to primary status.
[hb-3] I am not sure how a target server can initiate a migration process
I would imagine that on a fail-over or on a systematic-removal of a node
or any other such action that requires a cluster wide state-balancing, it is
the lb/adminS that would sense this first and initiate actions.
Any inbound request landing on a node that is not primary for the
required session results in a forward/redirect of the request to it's
current primary, or a migration of the session to the receiving node
and it's promotion to primary.
[hb-4] Not sure when this scenario would occur when a "secondary" would
receieve a HTTP request even when the primary is functionning well.
A shared store is used to passivate sessions that have been inactive
for a given period, or are surplus to constraints on a node's session
cache size.
[hb-5] yes this is very much a point that I have been saying since my
first proposal. Just to quote from that doc. "With a little intelligence
built in an MS can store away, less busy sessions to DB and retrieve them
when needed thus offering something that is near to "virtually unlimited
amount of sessions (section 1.1.1.1)"
Once in the shared store, a session is disassociated from it's primary
and replicant nodes. Any node in the cluster, receiving a relevant
request, may load the session, become it's primary and choose
replicant nodes for it.
[hb-6] this is a good optimization to sit on the scheme mentioned in [hb-5]
Correct tuning of this feature, in a situation where frequent
migration is taking place, might cut this dramatically.
The reason for the hard node-level session affinity requirement is to
ensure maximum cache hits in e.g. the business tier. If a web session
is interacting with cached resources that are not explicitly tied to
it (and so could be associated with the same replicant nodes), the
only way to ensure that subsequent uses of this session hit resources
in these caches is to ensure that these occur on the same node as the
cache - i.e. the session's primary node.
By only having one node that can write to a session, we remove the
possibility of concurrent writes occurring on different nodes and the
subsequent complexity of deciding how to merge them.
[hb-7] I complete agree
The above strategy will work for a 'implicit-affinity' lb (e.g. BigIP),
which remembers the last node that a session was successfully accessed
on and rolls this value forward as and when it has to fail-over to a
new node. We should be able to migrate sessions forward to the next
node picked by the lb, underneath it, keeping the two in sync.
With an 'explicit-affinity' lb (e.g. mod_jk), where routing info is
actually encoded into the jsessionid/JSESSIONID value (or maybe an
auxiliary path param or cookie), it should be possible, in the case of
fail-over, to choose a (probably) replicant node to promote to primary
and to stick
requests falling elsewhere to this new primary by resetting this
routing info on their jsessionid/JSESSIONID and redirecting/forwarding
them to it.
If, in the future, we write/enhance an lb to be Geronimo-aware, we can
be even smarter in the case of fail-over and just ask the cluster to
choose a (probably) replicant node to promote to primary and then direct
requests
directly to this node.
The cluster should dynamically inform the lb about joining/leaving
nodes, and sessions should likewise maintain their primary/replicant
lists accordingly.
[hb-8] I complete agree
LBs also need to be kept up to date with the locations and access
points of the various webapps deployed around the cluster, relevant
node and webapp stats (on which to base balancing decisions), etc...
All of this information should be available to any member of the
cluster and a Geronimo-aware lb should be a full cluster member.
On shutting down every node in the cluster all session state should
end up in the shared store.
These are fairly broad brushstrokes, but they have been placed after
some thought and outline the sort of picture that I would like to see.
Your thoughts ?
Jules
Bhagwat, Hrishikesh wrote:
I am also not convinced it reduces the amount of net traffic. After each
request the MS must write to the shared store, which is the same traffic as
a unicast write to another node or a multicast write to the partition
(discounting the processing power needed to receive the message).
I agree. However, this is based on the assumption that only one unicast
write is required. In other words, this is a primary/secondary topology. I
think that hd did not intended such a topology and hence his statement.
[hb] Yes i was not assuming a Pri/Sec design but a layout where any active
server
can be request to pick up a client request which is destined to server
that has just failed
-----Original Message-----
From: gianny DAMOUR [mailto:[EMAIL PROTECTED]
Sent: Sunday, October 19, 2003 7:35 AM
To: [EMAIL PROTECTED]
Subject: [Re] Web Clustering : Stick Sessions with Shared Store
Jeremy Boynes wrote:
However, as Andy says, the cost of storing a serialized object in a BLOB is
significant. Other forms of shared store are available though which may
offer better performance (e.g. a hi-av NFS server).
Do we need a shared repository or a replicated repository?
The issue I have with hb's approach is the reliance on an Admin Server, of
which there would need to be at least two and they would need to co-operate
between themselves and with any load-balancers. I think this can be handled
by the regular servers themselves just as efficiently.
I agree. It seems that in such a design an Admin Server is only used to
route incoming requests to the relevant node.
However, I do not believe that regular servers can do this job. I assume
that they will implement a standard peer-to-peer cluster topology to provide
redundancies, however I do not see how they can handle the dispatch of
incoming requests.
This feature seems to be either a client or a proxy one: I mean it should be
done prior to reach the nodes.
For instance, this feature is treated on the client-side via a stub aware of
the available nodes in WebLogic. It seems that JBoss (correct me if I am
wrong) has also followed this design.
I am also not convinced it reduces the amount of net traffic. After each
request the MS must write to the shared store, which is the same traffic as
a unicast write to another node or a multicast write to the partition
(discounting the processing power needed to receive the message).
I agree. However, this is based on the assumption that only one unicast
write is required. In other words, this is a primary/secondary topology. I
think that hd did not intended such a topology and hence his statement.
Gianny
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------------------------------------------------------------------------
Imagine a Cluster having N managed servers providing complete redundancy to each other. Imagine that they are all arranged in a cirlce.
They continue to be one big cluster till the Internal Network traffic (INT) is
less than a certain thereshold. At this stage any server in the cluster is just
as good as any other server in the cluster to provide fail-over support. This
scheme cannot however scale well largely due to 2 factors
1. As client requests increase the number of server-to-server exchanges for
Session replication would
increase, stressing the network.
2. There would be an added overhead for each server to process ALL of these
Sessions objects.
A ----------------- B ------------------- C ----------- D
| ----> |
| |
| |
J E
| |
| |
| |
I ----------------- H -------------------- G ----------- F
Here is a solution :
-------------------
After the INT crosses a certain threshold value, "A" stops sending Session-updates to "the server before
itself" (J). It also sends a notification to J to that effect. J can now forget all the session of A that it had stored up
untill now and thus get a chunk of free memory. Each server does the same thus "B cuts off A" and "C cuts off
B" and so on. The number of exchanges now reduces from (n * n) to (n * (n-1)). In case of failure "A" still has a
solid backup (All server from B to I).
After somemore increase in the traffic the servers "PERGES" yet another server. Thus this time "A"
will stop sending Session Objects to I (and ofcourse J). Follwing that it will send a notification (YOU_ARE_PURGED)to I
. On the other hand it "A" would have received a similar notification, this time, from C. However A still has
a strong enough backup (all servers from B to H) to fall on. Every machine holds with itself a list of ALL peers
(A-B-C- .. in the correct order) and the recent "cutoff-count (no. of servers purged - this value is obviously the
same for all the servers)".
Note that, in a way partitions are being created in the single large cluster. "A" OWNS a particition that originally contained B-J. It also is a member of 9 other particition owned respectively by B,C...J. As traffic and cliet-load increased A starts pushing out members of its partition thus making it smaller. In effect it gets removed from some other server's (say B .. for cutoff_count = 1 and from C when cutover_count = 2) partition.
As soon as a server is PURGED (goes out of a partition):
1. It need not any more receieve session-updates from the Partition OWNER -
saving network bandwidth
2. It need not remember any Session objects that were earlier given to it by
THAT partition owner - thus allowing it to have more free memory - to be
utilized for serving the increased client load.
Under this scheme, I think, the Cluster will have a fairly uniform turn around time
over a large range of "client load".
Even as all of this happens the LB will continue allocate NEW CLIENTS in a
simple round robin fashion while forwarding requests comming from all other
clients, who already have established session with a server, to their
respective servers.
Now we shall discuss 4 special cases
1. Failover
Now imagine that "A" (actully any randomly selected server) fails. When LB finds out that it is not able to reach
"A" it simple forwards the client request (destined for A) to the next available server in the list, "B". B tries to
serve the request but doesnt find the SessionID in its list of "NATIVE" sessions. THIS IS A SIGNAL TO "B" that
"A"HAS failed. Now it goes on to do the following CRUTIAL ACTIVITY :
It goes to its local Bucket that was dedicated to hold A's Sessions. It will scope out a collection of these for EACH SERVER IN THE CLUSTER (not just for the ones in the partition) ... So even if "A" owned a partition having A-B-C, yet the Session objects will be divided into 8 (B-J) sets. The SETS are now sent out to their respective owners. A special protocol (may be a different Queue/Topic) may be used to specify to the receipients that they are supposed to HEREAFTER treat these Sessions as being native to them (as if they have now being promoted as the Primary Node for the Sessions they just received). While B is doing this .. it may continue to get requests from A's clients (since LB simply forwards them). TILL SUCH TIME THAT B HAS NOT SENT OUT THE "SETS" IT WILL CONTINUE TO SERVICE SUCH REQUESTS. ALSO SUCH SESSIONS WOULD BE REMOVED FROM THE SETS AND WOULD THEREAFTER BE TREATED AS NATIVE TO B. After B sends out the SETS, it waits for acknowledgements from the servers. Before getting the ACK if it receives any more requests from A's clients, "B" will service that request and send the updated Session to the owner(say F). If after a stipulated amount of time "B" still doesnt receieve any ACKs it will distribute F's SET amongst the rest. For distribution it will again follow the same process. It will send F a signal to IGNORE its MAKE_NATIVE call (through which it had earlier handed over the Sesssions).
When a server acknowledges, B sends an UPDATE_LIST (sessions-newOWNER)to the LB. LB is hereon expected to
send the requests to the new OWNERS. "B" however continues to maintain this (UPDATE_LIST) list with
it. It will keep this list with it till the time LB does not ACK having received the UPDATE_LIST. The reason
to do so is in the folliwng special case : "B" has sent the SETs and the UPDATE-REPORT(LB) ... but
LB has not yet received it. Just then a request appears from A's client. LB still doesnt know about the new
ownership of this SESSION by another server ("D") and so continues with its policy to forwards it
to B. On receival B, finds out that it has received a call for A. It check for the correct owner of the
session and forwards the call to it.
By now all the Machines have shared the load of A and can continue serving all clients,
it is a good time now for the system to re-organize it self. what this means is that;
Partitions IJA and JAB are not complete (bocz "A" has failed) also partition
ABC no more exists. The seazure of ABC doesnt really matter but for the first two
partitions (IJA and JAB ) it means that their owners, respectively I and J, do not have
enough back up servers. Instead of two back up servers as every one else has, they have
just one available. Thus the SYSTEM needs to reorganize.
In the current example we have said that the cutover_count is 3. The system can take the new LIST OF SERVERS B-C-D-E-F-G-I-J and apply this cutover_count. BUT since one server has gone down they would each have to service more load thus the cutover_count is cut by ONE.
IMP NOTE: a system administrator can specify the MINIMUM and the MAXIMUM
value for cutover_count. LOWER THE CUTOVER_COUNT higher the performance (but
lesser the number of servers backing up one (each) server). On the other hand
you may have many servers backing up ONE (each) server .. thus assuring higher
failover capacity but at lower performance.
After getting ACK from all servers and all correspondign UPDATE-LIST-ACK from the LB, B will send out a RE_ORGANIZE signal with LIST as B-C-D-E-F-G-I-J and
cutover_count = (cutover_count > MIN_ALLOWED_CUTOVER) ? cutover_count
-1 : MIN_CUTOVER
to all the SERVERS
NOTE : In this case, all through out B is acting as an ADMIN SERVER. But its selection is
DYNAMIC. If server C would have gone down "D" would have been the ADMIN. This
design thus does not have a STATICALLY CONFIGURED ADMIN on whose failure the system
cannot run.
ANOTHER NOTE : if "C" receives a request that was ment for "A" (and NOT B)
then that means that both A and B failed (almost simultaneously). C must then execute the algo
explained above for BOTH the servers.
How the Cluster re-organizes (actually re-partitions), we shall see in the
next section. However one case needs to be explained before that. All the
discussion above pertains to the case where failover of A was follwed
immidiately by a client request for A. The following is a discussion on how the
system shall behave for requests comming in for other servers
1.> B,C,D,E,F,G,H (Servers which do NOT have A as a part of partitions they
OWN) : Processing happens as normal. Capability to back them up is not compromized
by A going down.
2.> I,J (Servers that DO CONTAIN A as a part of the partition they OWN) : After
processing the client request they shall find out, while trying to send the
updated SESSION to back-up servers, that one of their backup servers(A) has
failed. Since each server has a list of all peers (and in the correct order) they
can ask the server (B) following the one that failed (A) to take corrective
action. B will then execute the same process that is mentioned above ending
eventually with a call to RE_ORG. It is only after RE_ORG that I and J will have a
second back up to save their SESSIONS. Meanwhile they will continue to be backup
on ONE backup server that is still available.
Reorganization
--------------
Reorganization happens at all Servers simulataneously. It is a simple process.
A server (say G) receives a RE_ORG signal from B. Along with this signal it
receives the new SERVER_LIST (B-C-D-E-F-G-H-I-J) and then new cutover_count
(say 3-1 = 2). G will then undertake this two step process
1. Here after send all updates to the session (including the new ones it has just
received from B; A's sessions that B distributed to it) to ONLY its new member(s). In
this case "H".
2. It will send a YOU_ARE_PERGED signal to all other signals (meaning that they
are no a part of G's partition).
This process happens at each server. Thus J will have I as backup and I
will have B.
Note: If in case the MIN_ALLOWED_CUTOVER_COUNT is 3. B cannot send the new
cutover_count of (2). It will continue with the then current value of 3. thus
new partitions would be like GHI, HIJ, IJB, JBC and so on.
